Monitoring Scapular Kinematics through Wearable Magneto-Inertial Measurement Units: State of the Art and New Frontiers

Monitoring shoulder kinematics, including the scapular segment, is of great relevance in the orthopaedic field. Among wearable systems, magneto-inertial measurement units (M-IMUs) represent a valid alternative for applications in unstructured environments. The aim of this systematic literature review is to report and describe the existing methods to estimate 3D scapular movements through wearable systems integrating M-IMUs. A comprehensive search of PubMed, IEEE Xplore, and Web of Science was performed, and results were included up to May 2023. A total of 14 articles was included. The results showed high heterogeneity among studies regarding calibration procedures, tasks executed, and the population. Two different techniques were described, i.e., with the x-axis aligned with the cranial edge of the scapular spine or positioned on the flat surface of the acromion with the x-axis perpendicular to the scapular spine. Sensor placement affected the scapular motion and, also, the kinematic output. Further studies should be conducted to establish a universal protocol that reduces the variability among studies. Establishing a protocol that can be carried out without difficulty or pain by patients with shoulder musculoskeletal disorders could be of great clinical relevance for patients and clinicians to monitor 3D scapular kinematics in unstructured settings or during common clinical practice.


Introduction
The shoulder is a complex and kinematically redundant joint: it achieves great mobility through the interplay of three bones (clavicle, humerus, and scapula) and four joints (sternoclavicular, acromioclavicular, glenohumeral, and scapulothoracic) [1,2].
An accurate estimation of shoulder kinematics is an essential aspect in orthopaedic research due to the emerging need to improve clinical diagnostics, evaluate objectively post-surgery functional recovery, and help clinicians to customize the therapy by adapting it to the patient's specific needs [3][4][5][6][7]. One of the main obstacles in shoulder kinematic analysis is the difficulty of recording and assessing scapular motion non-invasively but accurately [8][9][10][11][12]. The three-dimensional kinematics of the scapulothoracic (ST) joint during dynamic movements can be described by rotations of the scapula on the thorax following the International Society of Biomechanics' (ISB) recommendations [13]. The scapula rotates in the frontal plane, resulting in a movement called Medio-Lateral Rotation

Literature Search Strategy and Eligibility Criteria
This systematic review was developed following PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines [40]. Articles were selected from three different databases, namely, PubMed, IEEE Xplore, and Web of Science. Free text terms and Mesh (Medical Subject Headings) were combined using logical Boolean operators (OR, AND). In each database, the search was executed as follows: ("shoulder kinematics" OR "shoulder joint" OR "shoulder biomechanics" OR "scapular biomechanics" OR "scapulothoracic joint" OR "scapular kinematics") AND (wearable* OR "wearable system*" OR "wearable device*" OR "wearable technolog*" OR "wearable sensor system*" OR "magneto-inertial sensor*" OR "magneto-inertial measurement unit*" OR "M-IMU*" OR "inertial and magnetic sensor*" OR "inertial-magnetic measurement unit*"). All studies published up to May 2023 were considered. The title and abstract of articles retrieved from the search were independently screened by two reviewers (C.A. and U.G.L.). In cases of disagreement, the final consensus for inclusion/exclusion was reached after discussion with two other reviewers (E.S. and A.C.). Articles were screened following the established inclusion/exclusion criteria.
In the selected articles, the upper arm sensor was placed on the posterior and distal end of the humerus, positioned halfway between the medial and lateral epicondyle [14,[17][18][19][21][22][23][24][25][26][27][28][29]. The forearm sensor was used only in a few articles, and it was placed dorsally at the distal end close to the ulnar process [18,19,23,[27][28][29]. Moreover, in the study of Höglund et al., two different positioning techniques to track upper limb motion were evaluated and compared: a total of four M-IMUs were placed centred on the lateral and dorsal distal end of both the upper arm and forearm [20]. In all the reviewed articles, the thorax sensor was placed on the chest under the jugular notch [35][36][37][38][42][43][44][45][46][47][48][49][50]. Its role was crucial because data from scapular M-IMUs are used for tracking rotational and translational movements of the scapula with respect to the thorax M-IMUs [17,51]. In the literature, two different positioning techniques were described for scapular M-IMUs ( Figure 2). The most common M-IMU positioning to track 3D scapular kinematics was to align the x-axis of the sensor with the cranial edge of the scapular spine [35,36,38,[42][43][44][45][46][47][48][49]. In particular, the scapula M-IMU was placed over the central third between the angulus acromialis (AA) and the trigonum spinae (TS) by palpating the scapular spine from the most lateral part of the acromion to the most medial aspect of the scapula [35,36,38,[42][43][44][45][46][47]49]. In the other positioning technique, the M-IMU for evaluating scapular rotation was positioned on the flat surface of the acromion with the x-axis of the sensor perpendicular to the scapular spine [37,50].
In the study of Cutti et al., M-IMU sensors positioned on the thorax, upper arm, forearm were mounted on rigid clusters of four markers [35]. In order to monitor 3D sc ular kinematics, a specific and lightweight cluster was fabricated. The clusters were t attached directly to the subjects' skin to track upper limb movements with both MOCA and wearable M-IMUs [35]. This solution ensured that the same negligible soft-tissue tifacts affected both tracking devices [35]. Following this experimental protocol, they tained a Root Mean Square Error (RMSE) between 0.2° and 3.2° for 97% of the angles d (i.e., elbow, humerothoracic, and scapulothoracic joint angles). These results show wearable M-IMUs' ability to provide angles data highly correlated to the MOCAPs dur the execution of bilateral upper arm movements [35]. Similarly, Grip et al. positioned IMUs on clusters showing high validity for scapular segments when compared MOCAPs (-0.2 ± 1.2°) [38].
Other studies placed sensors directly on the skin, and they demonstrated that ang trajectories were relatively similar between M-IMUs and MOCAPs [36,37,41,50]. H ever, there was a systematic error between the scapular kinematics reported from b motion capture systems [37,50]. This constant scapular angle offset was likely respons for some of the relatively higher RMSE values (RMSE = 25.6° for scapular IER during o head lift) [37].

Calibration
Sensor-to-segment calibration is an essential procedure for defining a relations between human body segments and M-IMUs [5,17,21]. For scapular monitoring, singl double and static or dynamic calibrations were proposed and investigated [52,53]. St calibration consisted of strictly aligning the axes of the sensors with those of the b segments [18]. During a dynamic calibration procedure, the subject performed a serie

Gold Standard
In six studies, the performance evaluation of wearable systems based on M-IMUs for monitoring scapular kinematics was compared with optoelectronic motion capture systems (MOCAPs) as the gold standard [35][36][37][38]41,50].
In the study of Cutti et al., M-IMU sensors positioned on the thorax, upper arm, and forearm were mounted on rigid clusters of four markers [35]. In order to monitor 3D scapular kinematics, a specific and lightweight cluster was fabricated. The clusters were then attached directly to the subjects' skin to track upper limb movements with both MOCAPs and wearable M-IMUs [35]. This solution ensured that the same negligible softtissue artifacts affected both tracking devices [35]. Following this experimental protocol, they obtained a Root Mean Square Error (RMSE) between 0.2 • and 3.2 • for 97% of the angles data (i.e., elbow, humerothoracic, and scapulothoracic joint angles). These results showed wearable M-IMUs' ability to provide angles data highly correlated to the MOCAPs during the execution of bilateral upper arm movements [35]. Similarly, Grip et al. positioned M-IMUs on clusters showing high validity for scapular segments when compared to MOCAPs (−0.2 ± 1.2 • ) [38].
Other studies placed sensors directly on the skin, and they demonstrated that angles' trajectories were relatively similar between M-IMUs and MOCAPs [36,37,41,50]. However, there was a systematic error between the scapular kinematics reported from both motion capture systems [37,50]. This constant scapular angle offset was likely responsible for some of the relatively higher RMSE values (RMSE = 25.6 • for scapular IER during overhead lift) [37].

Calibration
Sensor-to-segment calibration is an essential procedure for defining a relationship between human body segments and M-IMUs [5,17,21]. For scapular monitoring, single or double and static or dynamic calibrations were proposed and investigated [52,53]. Static calibration consisted of strictly aligning the axes of the sensors with those of the body segments [18]. During a dynamic calibration procedure, the subject performed a series of specific functional movements such as elbow flexion-extension, forearm pronation-supination, shoulder flexion-extension, and abduction-adduction [18,34]. In addition, the association between static and dynamic calibration, known as double calibration, was used [52,53]. The single calibration was the most used [35,36,38,42,44,46,47,50]. In several of the reviewed papers, the subject was instructed to stand still for a few seconds, with their shoulders and trunk in the neutral position, with both arms alongside the body, perpendicular to the ground, and with the thumb pointing forward [35,38,50]. In other studies, this static measure was performed with the subject standing in an upright position, the upper arm along the trunk for neutral humerus internal/external rotation, and the elbow flexed at 90 • [36,42,44,46,47]. A double static calibration procedure was performed by recording the scapular orientation in a neutral position and at maximum humeral elevation in both the sagittal and scapular planes [37,47].
A comparison of different calibration techniques was performed in one study [44]. Static posture with the trunk upright, the upper arm along the trunk, and the elbow in a 90 • flexion and double dynamic calibration were compared by van den Noort et al. [44]. The latter technique consisted of several static measurements performed at 0 • , 30 • , 60 • , 90 • , and 120 • of humerothoracic elevation [52]. Moreover, in the double calibration procedure, an active humeral anteflexion from 0 • to 120 • in the sagittal plane and humeral abduction from 0 • to 120 • in the frontal plane were recorded [52]. In conclusion, the authors suggested double calibration, especially in subjects with a high body mass index, to prevent the underestimation of scapular MLR [44].

Tasks Executed
Following sensor placement and completing the sensor-to-segment calibration, all the experimental protocols of the reviewed articles included the execution of several static positions or dynamic movements. The tasks most used for the estimation of threedimensional ST joint angles were flexion in the sagittal plane, abduction in the frontal plane, and elevation of the upper limb in the scapular plane [35][36][37][38][43][44][45][46][47][48][49]. Some studies assessed shoulder internal/external rotation [35,38,46,48]. Tasks relating to ADLs, such as hand to mouth, hand to neck, or hand to back, were also evaluated [35,38]. The experimental protocol of Friesen et al. included eight tasks of the WRAFT protocol [37]. The threedimensional ST joint angles were extracted during the execution of the elevation tasks (e.g., flexion in the sagittal plane, abduction in the frontal plane, comb hair, overhead reach, and overhead lift) [37]. Höglund et al.'s study evaluated subjects while performing nine standardized arm movements included in the modified Mallet scale [50]. During elevation movements (such as shoulder flexion-extension and abduction-adduction), only the scapular MLR and tilt were evaluated [50]. Scapular IER was assessed during the execution of tasks simulating ADLs [50].
Studies comparing the M-IMU-based wearable systems with MOCAPs obtained good to excellent results [35][36][37][38]41,50], although the results worsen as the complexity of the tasks increases [50]. In fact, for example, Grip et al.'s study demonstrated that, if comparing their results extracted by wearable M-IMU-based systems with the gold standard, the ICC values were close to 1 so they were high in reliability [38]. Similarly, in Friesen's study, an RMSE of 25.6 • and of 20.8 • in scapular IER during the execution of an overhead lift and abduction, respectively, were recorded with an M-IMU placed on the acromion [37]. In addition, this study aimed to assess which location (acromion or spine) was the best for the reliability of scapular motion. They found that the best positioning technique is to place the M-IMU on the acromion especially for IER across most tasks (minimum ICC recorded during overhead lift was of 0.670 compared to the corresponding value of ICC = −1.069).
The mean difference and standard error were evaluated by van den Noort et al., comparing two different calibration techniques [25]. The results showed high differences in the ROM and standard deviation between single and double calibrations (mean differences of −8.4 • , 14.4 • , and −12.1 • were recorded between scapular tilt, MLR, and IER) [25].
Other studies compare the scapular measurements recorded by two different operators (inter-reliability) [36,42]. The results were excellent especially for scapular IER (ICC = 0.80 during flexion and ICC = 0.78 during abduction) and MLR (ICC = 0.88 during flexion and ICC = 0.84 during abduction) in van den Noort et al.'s paper and for scapular tilt, MLR, and IER (CMC > 0.90 • ) during the execution of flexion and abduction in Parel's study [36,42].
Some articles aimed to compare the performance of different populations included in their studies [45]. Roldán-Jiménez's study investigated the differences in scapular threedimensional motions between young and older asymptomatic adults [45]. They found that subjects presented less scapular MLR and IER values when their age increases [45]. Other authors aimed to analyse the performance of patients [46,48]. For example, Reina et al. included a group of subjects undergoing reverse total shoulder arthroplasty (RTSA), and their work aimed to compare the pathological upper limb with the contralateral side [48]. The RTSA scapular results had high standard deviations (SD): in fact, SD values of 20.3 • and 6.1 • were recorded in scapular tilts during abduction in the pathological and healthy side, respectively [48]. Similarly, Ajčević et al. enrolled subjects with adhesive capsulitis [46]. They found a significant increase of the scapular ROM (mean ROM of 21 • and 34.6 • were extracted before vs. after rehabilitation treatments) [46].

Discussion
Advances in wearable systems based on M-IMUs have shown promising results in recording three-dimensional scapular motions. However, the lack of standardized protocols, sensor-to-segment calibration, and M-IMU positioning makes it difficult to identify a universal procedure that reduces the variability among studies.
Defining the best sensor positioning for scapular kinematics monitoring through wearable M-IMUs is crucial for providing clinically relevant information [42,50,56]. A protocol called ISEO (INAIL Shoulder and Elbow Outpatient protocol) has been developed and tested for estimate scapular kinematics through M-IMUs, showing a similar withinprotocol repeatability for the protocol that uses a scapula tracker (RMSE = 2.35 • , SEM = 2.5 • ) and the ISEO protocol (RMSE = 2.24 • , SEM = 2.3 • ) [35,41]. Höglund et al. demonstrated that positioning the sensor cranially along the scapular spine reduces the influence of underlying muscle and skin movements [50]. Despite this, Friesen et al. demonstrated that placing the scapular M-IMU on the acromion corresponds to a higher reliability than with spine placement [37].

Application in a Clinical Scenario
Patients suffering from shoulder joint dysfunctions may experience shoulder pain and restrictions in performing ADLs [57][58][59]. Due to difficulty performing free-pain movements, they often develop compensatory scapular movements to maintain a physiologic range of motion [60]. The quantitative evaluation of scapular kinematics with wearable M-IMUs in clinical scenarios represents a valid solution that complements subjective assessments using clinical scales [61][62][63][64].
Among the reviewed studies, some applied wearable systems integrating M-IMUs for monitoring scapular kinematics in children with brachial plexus birth injury, patients suffering from scapular dyskinesis, or patients undergoing reverse total shoulder prosthesis [38,44,48]. A bilateral configuration of wearable M-IMUs was used for evaluating 3D scapular rotations in 14 patients after primary reverse total shoulder arthroplasty [48]. The results showed a greater contribution of scapular movement in pathological shoulders than in the contralateral healthy shoulder (pathological vs. healthy side at 90 • -120 • of flexion (mean ± std): tilt, 28.9 • ± 7.5 • vs. 22.0 • ± 8.9 • ; MLR, 34.1 • ± 9.9 • vs. 31.4 • ± 13.0 • ; IER, −12.7 • ± 9.0 • vs. −8.7 • ± 8.6 • ) [48]. Similarly, for elevations in the frontal plane, these results showed how a greater ST contribution is essential to ensure the same level of humeral elevation [48]. In another study, patients with scapular dyskinesis were invited to perform elevations in the sagittal and frontal planes while the M-IMUs measured 3D scapular kinematics to investigate the effect of different calibration procedures on measurements [44]. Although the results showed that scapular locator calibration is necessary when using M-IMUs for scapular monitoring and that for elevations above 90 • , double calibration avoids the underestimation of scapular MLR, further investigation would be useful in more homogeneous populations to confirm the validity of the proposed method [44]. The validity of scapular motion monitoring was high when compared to MOCAPs (−0.2 ± 1.2 • ) during hand to neck, hand to spine, hand to mouth, and internal rotations performed by patients with brachial plexus birth injury, but the results should be cautiously interpreted given the undesirable contribution of underlying soft tissues [38].

Recommendations and New Frontiers
Placing the scapular M-IMU with the x-axis aligned with the upper edge of the scapular spine over the central third between the angulus acromialis and the trigonum spinae would seem appropriate. Firstly, as some authors suggest, positioning the scapular M-IMUs aligned the x-axis of the sensor with the upper edge of the scapular spine allowed for the defining of the scapular sensor units' system of reference axes as close as possible to ISB recommendations [13,35,36]. The proposed sequence was consistent with both research and clinical-based representations of scapular motions [13]. The three-dimensional scapulothoracic kinematics are then described by three independent angles obtained with the sequences of Euler angles: changing the sequence resulted in significant alterations in the description of motion [13,35,36]. Secondly, the positioning of the scapular M-IMU along the scapular spine was the simplest techniques of the methods described in the reviewed articles [50]. Simple yet reliable protocols and procedures should be further investigated to use wearable systems integrating M-IMUs to monitor scapular kinematics in patients with shoulder musculoskeletal disorders. The single calibration approach with the subject standing with a straight back and both arms along the body, perpendicular to the ground, could also be executed by patients unable to perform complete upper-limb movements or assuming a static position with the elbow at 90 • for several seconds because of pathological conditions. From this point of view, simplified segment-to-sensor calibration methods and sensor placements might allow for the easy monitoring of scapular kinematics during rehabilitation.
However, further studies should be conducted to establish a validated and universal protocol (including standardized M-IMU placement and calibration procedures). Establishing a protocol that can be carried out without difficulty or pain for patients with shoulder musculoskeletal disorders could be of great clinical relevance for patients and clinicians to monitor 3D scapular kinematics in unstructured settings or during common clinical practice, respectively.

Conclusions
Wearable systems equipped with M-IMUs are becoming a promising tool for evaluating the 3D scapular motion in orthopaedic clinical research. However, evaluating scapular kinematics by M-IMUs in patients with shoulder musculoskeletal disorders still presents open challenges to be overcome in order for them to be used systematically in clinical practice for optimizing patients' care.
Funding: This research received no external funding.

Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.

Data Availability Statement:
The datasets used and/or analysed in this study are available from the corresponding author upon reasonable request.

Conflicts of Interest:
The authors declare no conflict of interest.